Recovery of aromatic hydrocarbons



United States. Patent 0. l

3,201,489 RECOVERY OF AROMATIC HY DROCARBONS Donald Fred Knaack,Wilmington, Del., assignor to E. I. du Pont de Nemours and Company,Wilmington, lDeL, a corporation of Delaware 1 N Drawing. Filed Oct. 27,1960, Ser. No. 65,298 2 Claims. (Cl. 260-674) The present inventionrelates to the recovery of aromatic hydrocarbons from hydrocarbonmixtures containing such and more particularly to the separation ofaromatic hydrocarbons from mixtures thereof with saturated hydrocarbonsand the separation of specific aromatic hydrocarbons from mixtures ofaromatic hydrocarbons.

, It had heretofore been known that olefinic hydrocarbons could beseparated from hydrocarbon mixtures containing such by passing thehydrocarbon stream through a silver fluoborate solution. The fluoboratereacts with the olefin to form a soluble complex. Saturated hydrocarbonsare not affected by the silver fluoborate and pass through the solutionunchanged. The olefin silver fluoborate complex formed is unstable andis readily decomposed at elevated temperatures and/ or reducedpressures, releasing the olefin. The complex can also be decomposed byextreme dilution through addition of water. The reac tion of the silverfluoborate with theolefin is extremely rapid and proceeds to anextremely high degree of completion, so that the separation of theolefin is very effective. The separated olefin, furthermore, has apurity of greater than 99%. However, it was heretofore believed that theseparation was only etfective with olefins, i.e., compounds containingan ethylenic type of double bond, since the operability of the processwas based on the rapid reaction of the olefinic double bond with thesilver fluoborate. I

It is an object of the present invention to separate aromatichydrocarbons from hydrocarbon mixtures containing such. 'It is'anotherobject to separate mixtures of aromatic hydrocarbons. A further objectis to separate fluid mixtures of aromatic hydrocarbons and olefinichydrocarbons. Still another object is to provide a process for theseparationof aromatic hydrocarbons from mixtures containing such usingselective complex formation. Other objects will become apparenthereinafter.

In accordance with the present invention, fluid aromatic hydrocarbonsare separated from fluid mixtures of aromatic hydrocarbons differing inabsorptivity, fluid mixtures of aromatic hydrocarbons and saturatedhydrocarbons and fluid mixtures of aromatic and olefinic hydrocarbons bycontacting the hydrocarbon mixture with an aqueous solution of a silversalt selected from the class consisting of silver fluoborate, silverfluosilicate and mixtures thereof, and thereafter recovering thearomatic hydrocarbon absorbed by the solution.

In a preferredembodiment of thepresent invention, the silver saltsolution is modified by the addition of a stable metal salt wherein thecation has a charge to ionic radius ratio of greater than one andwherein the anion is selected from thegroup consisting of fluoborate,fluosilicate and mixtures thereof. The ratio of valence or charge toionic radius is readily calculated from published data. Thus,

both can be found'in Therald Moellers textbook, In-

organic Chemistry, published by John Wiley and Sons, Inc., 1952, onpages 140 to 142. In particular, the metals 3,2hlA89 Patented Aug. 17,1965 in Group H of the Periodic Table of Elements found in said textbookhaving atomic numbers from 4 to 56 inclusive, and copper, lead andlithium, are highly suitable. The molar ratio of silver to the secondarymetal is not critiIcal butis preferably maintained between 1:1 and 10:

The concentration of the silver salt is not critical but generallyconcentrated solutions of 4 to 12 molar are preferred, since theseparation is more complete at these concentrations. For certainseparations, however, it may be preferred to employ lower concentrationsof the silver salt. The preferred anion of the silver salt or thesecondary metal salt is fluoborate because of its higher activity in thesystem of the present invention. The anion of the silver and thesecondary metal salt need not be the same and it is quite feasible tocarry out the process with mixtures of anions in the aqueous solution,i.e., mixtures such as fluoborate and fiuosilicate.

The process of the present invention is preferably employed .in theseparation of fluid aromatic hydrocarbons from fluid hydrocarbonmixtures containing aromatic hydrocarbons admixed with saturatedhydrocarbons, i.e., saturated aliphatic and/or cycloaliphatichydrocarbons. Olefinic hydrocarbons, i.e., hydrocarbons which containnon-aromatic unsaturation, also form complexes with the silver saltsolutions. The absorptivity of such olefins differs from aromatichydrocarbons. Depending on the concentration of the silver salt solutionit is possible to either preferentially absorb the olefin orpreferentially absorb the aromatic hydrocarbon, e.g., in a mixture ofcyclohexene and benzene cyclohexene is preferentially absorbed at lowconcentrations (l- -4 molar silver fluo borate), Whereas at highconcentrations (6-12 molar silver fluoborate) benzene is preferentiallyabsorbed. The ef fect of concentration will shift somewhat with theaddition of the secondary metal salt. Thus by proper design it is alsopossible to separate olefinic hydrocarbons from aromatic hydrocarbons.The term fluid is used through out to denote a liquid or gaseous stateof the material at the temperature employed in the absorption step. Theterm aromatic hydrocarbon as employed in the description of thisinvention denotes hydrocarbon compounds containing no otherunsaturation. Hydrocarbons containing ethylenic unsaturation are definedas olefinic hydrocarbons. The compounds principally separated by theprocess of the present invention are therefore benzene, toluene,xylenes, and fluid homologs and isomers of these homologs. The processof the present invention is particularly-applicable to the separation ofthese aromatic hydrocarbons from mixtures with saturated aliphatic and/or cycloaliphatic hydrocarbons which are difficult to separate bydistillation becauseof close boiling points or formation of azeotropeswith the aromatic hydrocarbons, e.g., aliphatic and cycloaliphatichydrocarbons having from 6 to 12 carbon atoms. The process isfurthermore of utility where the aromatic hydrocarbon occurs only insmall concentration in a hydrocarbon stream and it is ability to beabsorbed by the salt solution. As will be apparent hereinafter, aromatichydrocarbons differ in their ability to be absorbed depending on theconcentration of the absorbing solution. Thus, at low concentrations,xylenes are not absorbed whereas benzene and toluene are. Hence, thisdifference in absorption provides a method for separating aromatichydrocarbons from each other. Where the absorptivity is the same, itwill be apparent that such aromatic hydrocarbons cannot be separated bythe process of the invention claimed.

More than one hydrocarbon can be separated when simultaneously admixedwith saturated hydrocarbons. Thus, both benzene and toluene can beseparated from hydrocarbon mixtures containing same in a single passthrough the absorber.

The process of the present invention is characterized by extremesimplicity of operation in that it is only necessary to bring thehydrocarbon mixture in contact with the aqueous solution containing thesilver salt for a suificient length of time to allow the formation ofthe complex. Where the hydrocarbon mixture is gaseous at absorptiontemperatures it is passed through an absorption tower containing theaqueous solution which may be continuously regenerated or usedbatchwise. If the hydrocarbon mixture is liquid at the absorptiontemperature, the separation can readily be carried out in an agitatedvessel in which the hydrocarbon mixture and the aqueous salt solutionare admixed for a sufiicient time to allow transfer of the aromatichydrocarbon into the aqueous phase. The absorption of the aromatichydrocarbon is normally carried out at room temperature; however, it ispossible to employ temperatures in the range of to 50 C. Desorption isaccomplished by heating the aqueous solution, reducing the pressure overthe aqueous solution, or by a combination of both. Suitable desorptiontemperatures at atmospheric pressure are 80 C. or higher. At reducedpressures, the use of lower temperatures will result in substantiallycomplete recovery of the absorbed aromati hydrocarbon. 1

Certain impurities, such as carbon dioxide, carbon monoxide, oxygen,hydrogen, nitrogen or noble gases, have only a very small elfect on theoperability of the described process. Where such impurities become majorcomponents of the mixture to be separated, it may be desirable to removethese impurities in a prior step. It has been found that theconcentration of acetylene should be maintained at a low level in orderto prevent interference of silver acetylide with the separation processpracticed. Concentrations of acetylene should remain on the averagebelow 1%. Where the acetylene in the gas to be separated is continuouslyat levels above 1%, it is preferred to hydrogenate the acetylene tosaturated compounds which do not interfere with the separation prior tocontacting the hydrocarbon stream with the salt solution.

The following tables show the absorption of various aromatichydrocarbons by the silver salt solutions employed in the process of thepresent invention. The absorption was determined by agitating a mixtureof the salt solution and the aromatic hydrocarbon at 24:1" C. until theamount of the aromatic hydrocarbon absorbed reached a constant value.

Table I.Abs0rpti0n of benzene absorbed by the silver solution.

Table II.Absorpti0n of toluene Moles of Toluene/ Mole of SilverAbsorption Solution AgBF4 AgBF4-L8 1W (B154) A ar.

Table III Moles oi Xylene/ Liter oi Solution Moles of Xylene/ Mole ofSilver Aromatic Absorption Solution Hydrocarbon M AgBFq M AgBF4-1.8 MMg(BF4)2.

AgBF

o-Xylene a NlO .4 M AgBFq No measurable absorption of either ortho-,meta-, or para-xylene occurred when 2.0 molar silver fluoboratesolutions, 2.0 molar silver fiuoborate-1.8 molar magnesium fluoboratesolutions, or 4.0 molar silver fluoborate solutions were employed.Hence, the difference in absorptivity of benzene and xylenes at 2 to 4molar concentration of silver fluoborate provides a method for theseparation of benzene from mixtures of benzene and xylenes.

The separation of benzene from a mixture comprising 2% benzene, 49%cyclohexane and 49% n-hexane (by volume) is shown in Table IV. Thehydrocarbon mixture, 20 ml., was agitated with 30 ml. of the silversolutions shown in Table IV for 5-15 minutes at 24 C. In each of theseparations, no cyclohexane or n-hexane was Ultravioletspectrophotometry was employed to determine the amount of aromatichydrocarbon absorbed.

Table IV Absorption solution: Percent of benzene absorbed 5.9 M AgBF, 195.9 M AgBF -1.8 M Mg(BF 50 11.8 M AgBF 78 Greater than 90% of thebenzene absorbed could be recovered from the absorption solution byheating the solution to 99 C. and distilling off the benzene. It will beapparent that with more efiicient equipment, such as countercurrentabsorption towers, much higher percentages of the benzene will beextractable.

The foregoing separation was repeated using the following absorptionsolutions:

Table V Absorption solution: Percent of benzene absorbed N 4 5.8 N AgSiF -L8 M Mg(BF 18 11.6 N Ag SiF 13 The separation of benzene andcyclohexene from cyclohexane and the preferred absorption of oneunsaturated compound as compared to the other unsaturated compounddepending on the concentration of the absorbing solution is shown inTable VI. A hydrocarbon mixture containing 10% cyclohexene, 10% benzene,and of cyclohexane, 20 ml., was agitated with 10 ml. of the silver saltsolutions shown in Table V1 for 5 to 15 minutes at 24 C. In each of theseparations no cyclohexane was absorbed.

' Table VI Absorption Solution Grams Grarns/ Amount of Material AbsorbedMoles/ ml. lite liter X X x N Cyclohexone 0. 12 12. 0 0. 10 MAgBF{cenlzelilie 0. 843 4. 3 0. 055 yo 0 exene 0. 97 9. 7 0.12 10 M AgBFenzene o. 24 24. 0 0.31 steers: ti? as as? Cyclohexene O. 34 34. 0 0. 4110 M AgBm {Benlzekllie 0. 86.0 1. 10 yo 0 exene 0. 48. 0 0.59 10 M AgBFBenzene 2 47 247. 0 3.17 e The amount of benzene p1 esent was determinedby ultraviolet spectrophotometry The results show the preferredabsorption of cyclo- I claim:

hexene using concentrated solutions of silver fluoborate. Table VIIillustrates the absorption of benzene using taining, by volume percent,2% benzene, 2% mesitylene (1,3,5-trimethylbenzene), 48% n-hexane, 48%cyclohexane. To ml. of the absorption solution shown in the table wasadded 20 ml. of the mixture described above. The mixture was shaken for15 minutes and the Ultraviolet spectrophotometry was used to determinethe N0 mesitylene,

Substantially all of the absorbed benzene could be recovered by heating.

aromatic hydrocarbon is desorbed, and the stripped silver solution isrecycled to the absorption tower.

The process set forth in claim 1 wherein the salt solution comprisessilver fluoborat-e modified with mag- References Cited by the ExaminerUNITED STATES PATENTS 1/49 Friedman et al 260 677 2,913,505 11/59 VanRaay et al. 260-677 2,953,589 260674 X 260-677 OTHER REFERENCES TolueneSoluble Copper and Silver Fluoborates, by J. C. Warf, J. AmericanChemical Society, vol. 74, p. 3702-3703 relied on, 1952.

Complex Fluorides, The Properties of Silver Salts and ALPHONSO D.SULLIVAN, Primary Examiner.

1. A PROCESS FOR SEPARATING FLUID AROMATIC HYDROCARBONS FROM FLUIDMIXTURES OF AROMATIC HYDROCARBONS WITH SATURATED HYDROCARBONS, WHICHCOMPRISES CONTACTING SAID MIXTURE WITH AN AQUEOUS SOLUTION OF A SILVERSALT WHEREIN THE ANION IS SELECTED FROM THE CLASS CONSISTING OFFLUOBORATES, FLUOSILICATES AND COMBINATIONS THEREOF, AND WHEREIN THESILVER SALT SOLUTION IS MODIFIED BY THE ADDITION OF A SECOND METAL SALTWHEREIN THE CATION IS A METAL OF GROUP II OF THE PERIODIC TABLE OFELEMENTS AND HAS AN ATOMIC NUMBER FROM 4 TO 56 INCLUSIVE, AND THE ANIONIS SELECTED FROM THE GROUP CONSISTING OF FLUOBORATES, FLUOSILICATES ANDCOMBINATIONS THEREOF, AND THEREAFTER RECOVERING THE AROMATIC HYDROCARBONFROM SAID SOLUTION.